Unable to insert comment before document element

Hello,

As far as I know there is no objection against having a comment between the 
document type definition and the document element.
So why can't I insert a comment before an existing document element with 
xerces. 

                Element root = doc.getDocumentElement();
                doc.insertBefore(root, doc.createComment("My Comment"));

I've tried to this with version 2.8.1.

I hope someone can explain.

Regards,

Henk Spaan